As known to the applicant, the existing gas instantaneous water heaters all have the problem of “coldwater sandwich”, i.e., during the usage of the hot water, when the user turns on the water again in a short time after it is turned off, the outlet water temperature of the water heater fluctuates due to the reasons such as the ignition preparation delay during startup, the water temperature rise during shutdown, etc., and especially the water temperature drops suddenly. This phenomenon is particularly apparent in winter, and the user's comfort in using the water heater is greatly influenced. In order to solve the problem, most of the water heater manufacturers adopt auxiliary heating measures such as utilizing the recycled water by adding a water pump, or connecting the water heater in series with a small electronic water heater which is often referred to Little Kitchen Aid, but the user's investment cost is obviously increased.